A reliable method of rejecting unwanted thermal energy to ambient is frequently required, for example, as part of many industrial and manufacturing processes, and in occupied building heating, ventilation and cooling (HVAC) technologies. This requirement is present in both temperate and tropical climates. Discharge of thermal energy to the ambient environment can be achieved by passing thermal transfer fluid through a conducting and/or radiating heat exchanger surface, where the temperature of the fluid is greater than ambient dry bulb temperature.
Where the transfer fluid temperature is close to ambient dry bulb temperature, a relatively large heat exchanger surface is required to discharge the required amount of thermal energy. Where ambient temperature exceeds the temperature of the transfer fluid, no advantageous transfer of thermal energy can take place. In these circumstances, use of a heat pump (reverse Rankine cycle) is commonly used to raise the temperature of the transfer fluid well above ambient, so that transfer of unwanted thermal energy to the environment can readily take place by radiation, convection and or conduction, before cooling the fluid by expansion to a temperature suitable to provide the necessary cooling effect.
As an alternative to radiation or convection transfer through a heat exchanger, direct evaporation of a transfer fluid, commonly water, may be used to transfer energy to the ambient environment. Direct evaporation involves transferring the latent heat of evaporation of water or other fluid vapour from a body of liquid, into the ambient air, with a corresponding fall in the sensible heat energy in the remainder of the body of liquid resulting in a temperature reduction. This method is used, for example, in cooling towers, typically combined with a heat exchanger to transfer thermal energy from a transfer fluid, in the heat exchanger, to the evaporating water droplets, resulting in a fall in the temperature of the transfer fluid.
One advantage of evaporative cooling is that thermal energy transfer can continue even below the ambient dry bulb temperature, even to as low as the wet bulb temperature, which may be ten or more degrees Celsius lower than dry bulb temperature in some instances, thus increasing the temperature differential across the heat exchanger in the cooling tower. Where temperature transfer can be achieved at a higher temperature differential than available from the ambient dry bulb temperature, a smaller heat exchanger may be used, for a given thermal load. Alternatively, less energy may by consumed by a reverse Rankine cycle heat pump that is using the transfer fluid from a cooling tower as a heat sink, to achieve the required transfer ofunwanted thetmal energy to ambient. However, with indirect contact cooling tower technology, heat transfer across a heat exchanger always results in a temperature of the transfer fluid at least several degrees above the prevailing wet bulb temperature.
The use of a cooling tower alone is ineffective in many climates as the wet bulb temperatures do not fall low enough, for sufficiently long periods of time in summer, to produce a usefully cooled transfer fluid. Where temperature differences are small, very large surface areas and very large flow rates are required, necessitating greater energy to operate the cooling tower in the form of high exergy electrical/mechanical energy, typically supplied by fossil fuel powered electrical generators or other electrical generators.
The other commonly used technology, the reverse Rankine cycle requires even greater input of high quality electrical/mechanical energy (high exergy), to remove low quality thermal energy (low exergy), resulting in a high degree of irreversible conversion of energy (loss of exergy) typically requiring the burning of fossil fuels or a nuclear power cycle to reliably provide electricity for the heat pump. There is a widespread desire to reduce the use of these fuels in energy cycles.
It is an object of the present invention to overcome at least some of the aforementioned problems or to provide the public with a useful alternative.
Any discussion of documents, acts, materials, devices, articles or the like, which has been included in the present specification is solely for the purpose of providing a context for the present invention. It should not be taken as an admission that any or all of the previous discussion forms part of the prior art base or was common general knowledge in the field of the invention as it existed before the priority date of any of the claims herein.